1. Field of the Invention
The invention relates to a disk, particularly a magnetic disk, to a method for making the disk, and to a disk drive unit with one or more such disks.
2. Background Information
An annular-shaped substrate that carries a magnetic coating on one or both sides is commonly called a magnetic disk. Magnetic disks are typically used in disk drive units in computers, for example, for data storage applications. The substrate of a magnetic disk may be formed from aluminum or from glass, for example, and the magnetic coating carried by the substrate may be deposited not on the substrate's surface itself, but on a base coating interposed between the substrate and the magnetic coating. It is known to use glassy carbon, amorphous carbon, or a metal or metal alloy for such a base coating. The magnetic coating may be a multi-layer coating. A typical example would be a seed layer, followed by a chrome layer, followed by a cobalt-platinum-chrome layer, followed by a protective layer and a lubricating layer.
In a typical disk drive unit, one or more magnetic disks are mounted on a spindle that is rotated by a motor. Magnetic heads that are movable with respect to the magnetic disks interact with the magnetic coatings to read and write information. The heads are generally so-called “flying heads” that glide aerodynamically over the surfaces of the magnetic disks on thin films of moving air generated by the magnetic disks when they rotate. As a general rule, the information storage density increases when the flying distance between the heads and the surfaces of the disk decreases. However, the risk of a so-called “head crash,” when a head actually touches the surface of a magnetic coating or possibly gouges into it, usually caused by the presence of asperities, also increases as the flying distance decreases.
It is desirable for each surface of a magnetic disk that carries a magnetic coating to be extremely flat and smooth. The reason is that a wavy surface or a rough surface that creates turbulence would make it necessary to either increase the flying distance, and thus reduce the recording density, or to increase the risk of a head crash and thus increase the prospect of losing data or physically damaging the disk drive unit. For contact recording, an asperity free, smooth surface is even more important.
It has been found that aluminum substrates are limited in how smooth their surfaces can be made. Accordingly, increasing attention has been directed to the utilization of glass substrates. Glass substrates are typically made from soda-lime or aluminosilicate glass. A manufacturing process known as the fusion process produces glass substrates with particularly smooth surfaces.
Typically, a manufacturer of disk drive units purchases glass blanks for use as substrates from a glass manufacturer. The manufacturer then subjects the glass blanks to various processes to prepare them for receiving the magnetic coatings. One of these processes is polishing, and what is known as chemical-mechanical polishing (or CMP) is frequently used. In this technique, a polishing slurry that contains very fine abrasive particles, whose surfaces depolymerize the glass chemically as they also mechanically polish the surface, is rubbed against the glass blanks, usually by a polishing machine designed for this purpose.
One drawback of CMP is that the used slurry, with its abrasive particles and glass residue, is relatively expensive to dispose of in an environmentally responsible manner. Additionally, cleaning the polished substrates is relatively costly since every last bit of abrasive grit and glass residue must be removed. Furthermore, it would be desirable for the magnetic coatings of magnetic disks to lie on asperity-free, smoother surfaces than can be achieved by CMP.
Ion beam technology has long been used, particularly in the semiconductor industry, for depositing thin films and for doping semiconductor wafers. This conventional technology uses what might be called monomer ions, or isolated charged particles. More recently, gas cluster ion beam technology has attracted attention for possible use in cleaning and smoothing surfaces and for oxidizing surfaces. This technology employs charged gas clusters rather than monomer ions. By using gas clusters rather than monomer ions, a lower kinetic energy per particle can be achieved. The use of gas cluster ion beam technology to smooth a sapphire wafer or glass surface is known. Magnetic heads are also a known application for this technology.